The present invention relates to a coating method (or bonding method) which is useful in coating a base material with an intermetallic compound or bonding base materials to each other with an intermetallic compound and relates to a coating apparatus therefor. More particularly, the present invention is concerned with a technology which can be applied to, for example, a corrosion resistant coating provided on a turbine blade of aircraft engine, a heat resistant, wear and abrasion resistant coating provided on an exhaust valve of automobile engine and a corrosion resistant, oxidation resistant coating provided on an exhaust system of garbage incinerator and further can be applied to a buildup welding for repairing parts thereof.
It is inevitable to develop a light material of high heat resistance as a structural material for use in heat engines accommodated in next-generation automobiles, marine vessels, aerospace equipment and other transport means and also accommodated in thermomotor of energy plants. Intermetallic compounds such as aluminides expected as providing a heat resistant material of high strength are materials which can meet the above requirements, and investigations for putting the intermetallic compounds to practical use, including those on the composition, texture optimization, performance enhancement and processing method for obtaining a desired structure, are being conducted in a number of countries.
It is common practice to coat the surface of a metal material with an intermetallic compound in order to impart a corrosion resistance and a wear and abrasion resistance thereto. However, the principle and method of simply obtaining a coating of intermetallic compound have not yet been established. As the method for obtaining an intermetallic compound, there can be mentioned the diffusion process, the flame spraying process and the combustion synthesis process.
The diffusion process is a process that after another type of metal which can react with the base metal to thereby form an intermetallic compound is applied to the surface of a base material metal by, for example, vapor deposition, the base material is heated and maintained in an elevated temperature, and whereby a reaction layer is formed on the base material surface. In the diffusion process, the diffusion, namely ultralow-velocity mass transfer phenomenon is utilized. Therefore, although there is such an advantage that controlling of the coating thickness of reaction layer is easy, there occurs such a disadvantage that the period of time required for coating formation is as extremely long as several hours to tens of hours. Accordingly, the attained coating thickness is restricted to a small one such as tens of microns or less. Further, the processing is performed at high temperature, so that the effect thereof on the base material cannot be disregarded.
The flame spraying process is a method that, as disclosed in, for example, Japanese Patent Laid-open Publication No. 2000-96206, a layer of alloy containing a specified type of element is piled on a base material by vacuum plasma spraying and thereafter aluminum is thermal-sprayed over the deposited alloy layer so that a coating layer of intermetallic compound is formed by a metallurgical reaction accompanied by heat buildup of aluminum. It is advantageous in that extreme energy consumption is not required because an exothermic reaction can be induced at a relatively low temperature within a relatively short period of time, as compared with the process of directly melting an intermetallic compound at high temperature. However, this process necessitates a large heat source of, for example, plasma, electric arc or laser. The flame spraying process poses such an inherent problem that voids are likely occur in the coating because of the adherence between the coating layer and the base material and because of the entrainment of ambient gas at the flame spraying. Furthermore, the thickness of the coating layer formed by the above process only ranges from about tens of microns to hundreds of microns, even if the alloy layer and the aluminum layer are summed up, because of spraying workability and cost, etc. As a result, there is the danger of cracking or local exfoliation caused by the nonuniformity, brittleness, etc. of formed coating.
The self-propagating high temperature synthesis process, or combustion synthesis process is a process wherein a reaction of compound formation spontaneously proceeds with the generation of high heat of reaction within a short period of time. The process is characterized in that a high-melting-point ceramic or intermetallic compound can be easily synthesized. Since the principle of the combustion synthesis process was discovered by Meljarnov, et al. of the old Soviet Union in 1967, theoretical researches and application developments have been promoted (see Journal of the Metallurgical Society of Japan, vol. 32, No. 12, page 845). The combustion synthesis process is a process that a green compact of a mixture of different types of metals capable of forming an intermetallic compound is placed on a base material surface and the base material is heated under pressure so that a combustion synthesis reaction is induced to thereby accomplish not only formation of an intermetallic compound but also bonding of the intermetallic compound to the base material. The combustion synthesis process, because a self-exothermic reaction is utilized, has such an advantage that the synthesis of coating layer and the bonding thereof to the base material can be simultaneously performed within a short period of time. However, because of a balance between the exothermicity at the synthesis and the heat transfer to the base material, the thickness of the coating layer is limited to large one such as several millimeters or more.
Moreover, the intermetallic compound such as NiAl or TiAl can be utilized at high temperatures such as 800 to 1000xc2x0 C. and exhibits superior properties as a high-temperature material. Therefore, development researches for the application thereof to engine parts, aerospace mechanical parts, etc. are being promoted, but, despite the excellent properties, ductility required for molding and working cannot be obtained to thereby inhibit the practical use thereof. The intermetallic compound has a high formation energy of about 100 kJ per mol, and the combination of metals is accompanied by an exothermic reaction of high temperature such as 1500 to 2000xc2x0 C. to thereby enable spontaneous proceeding of a synthetic reaction. Therefore, the combustion synthesis process is one of energy saving not requiring any external heating at high temperature, but controlling of reaction propagation, temperature and texture is difficult. As a result, many problems remain unsolved on the application of intermetallic compound to parts requiring heat resistance. For example, in the conventional combustion synthesis process, different types of metals are mixed together and used in the form of a green compact, so that the different types of metals are simultaneously heated at the same temperature. Consequently, the melting point of one of the metals automatically becomes the reaction initiation temperature, so that the reaction initiation temperature, thus the exothermic temperature resulting from the reaction, and the melt depth of the base material cannot be controlled. In the event of a thin coating layer, it may occur that the bonding strength cannot be enhanced.
As apparent from the above, although the coating of base materials with the intermetallic compound can be effected by, for example, the diffusion process, the flame spraying process and the combustion synthesis process, numerous steps and much energy consumption are inevitable because of the property of the intermetallic compound that working thereof is extremely difficult. Thus, coated members (for example, buildup welded members) cannot be obtained with less energy consumption at low cost within a short period of time.
Therefore, it is an object of the present invention to provide a method of not only simply and efficiently coating a base material with an intermetallic compound but also welding a plurality of base materials so as to bond them together and to provide a coating apparatus employed therefor.
It is another object of the present invention to provide a method of forming a coating layer of intermetallic compound having a thickness of hundreds of microns (xcexcm) to several millimeters (mm) with reduced energy consumption within a short period of time, and to provide a coating apparatus employed therefor.
It is a further object of the present invention to provide a method of easily controlling a reaction initiation temperature and, irrespective of the thickness of a coating layer, increasing the bonding strength of the coating layer to a base material, and to provide a coating apparatus employed therefor.
It is still a further object of the present invention to provide a method of not only rendering preprocessing (for example, vapor deposition in the diffusion process, piling up of a precursor layer in the flame spraying process and fabrication of a green compact in the combustion synthesis process) unnecessary but also freely and automatically performing coating of a base material surface with an intermetallic compound with the use of a control system (for example, CAD/CAM system) enabling computerized fine control.
It is still a further object of the present invention to provide a method of forming a coating layer of intermetallic compound in an energetically advantageous manner wherein a heat source of relatively low capacity (for example, high-frequency heater or resistance heater) can be employed.
The inventors have found that, when a first substance is piled up on a base material, and a second substance is delivered onto the first substance and reacted therewith, the thus formed intermetallic compound is fusion united with the base material to thereby enable formation of a coating layer of intermetallic compound on the base material. As a result, the present invention has been completed.
Therefore, according to one aspect of the present invention, there is provided a method of forming a coating layer of an intermetallic compound on a base material, comprising the steps of piling up a first substance on a base material, and delivering a second substance onto the first substance, the second substance reacted with the first substance to thereby form a coating layer of an intermetallic compound on the base material. The present invention is also useful for providing a method of welding a plurality of base materials to each other with an intermetallic compound. Therefore, according to another aspect of the present invention, there is provided a method of welding a plurality of base materials to each other with an intermetallic compound, comprising the steps of piling up a first substance on base materials, and delivering a second substance onto the first substance, the second substance reacted with the first substance to thereby cause the plurality of base materials to be bonded to each other through a coating layer of an intermetallic compound. In these methods, the coating layer may be fusion bonded to each base material so that a building up coating layer is formed on the surface of the base material.
In the methods of the present invention, the first substance can be constituted of, for example, at least one metal selected from the group consisting of nickel, cobalt, iron, niobium, vanadium, molybdenum, tungsten, chromium and tantalum. The second substance can be constituted of, for example, at least one metal selected from the group consisting of aluminum and titanium. The substance constituting the base material or base materials may be any of various metals, for example, a metal or alloy of at least one member selected from the group consisting of iron, nickel, cobalt, aluminum and niobium. Further, in order to increase the bonding strength of the coating layer to the base material, the substance constituting the base material may be at least one metal selected from the group consisting of metals of the first substance and the second substance, or a metal homologous thereto.
The methods of the present invention can be accomplished in various modes, for example, a mode wherein the first substance is piled up in powdery form or molten form on the base material or base materials, and the second substance is delivered in molten form or powdery form onto the piled first substance. In this mode, at least one of the first substance and the second substance is used in molten form.
The first substance may contain a ceramic. In particular, the first substance in powdery form or molten form may contain a powdery or fibrous ceramic constituted of an oxide, carbide, nitride or boride of at least one metal selected from the group consisting of aluminum, yttrium, titanium, zirconium, hafnium and silicon. In the present invention, a coating layer constituted of an intermetallic compound, an intermetallic compound having a ceramic dispersed therein or an intermetallic compound containing a nitride can be formed by the reaction between the first substance and the second substance.
In a further aspect of the present invention, there is provided a method of preparing a three-dimensional molding (or three-dimensional model, or three-dimensional shaped item) with the use of a computerized control system, comprising the steps of piling up a portion of first substance on a base material, and delivering a portion of second substance onto the piled first substance to thereby form a layer of intermetallic compound; and piling up another portion of first substance on the intermetallic compound layer, and delivering another portion of second substance onto the piled first substance to thereby form another layer of intermetallic compound.
The coating apparatus of the present invention is an apparatus for forming a coating layer of intermetallic compound on a base material by the use of a reaction between a first substance and a second substance, which coating apparatus comprises a unit for piling a first substance on a base material, a unit for delivering a second substance onto the first substance, and a unit for melting at least one of the first substance and the second substance.
In the present invention, because the exothermic reaction of materials per se is effectively utilized, not only the intermetallic compound optimized with respect to the composition and texture can be formed but also the coating layer of intermetallic compound (especially, high-melting-point dense welding coating layer or buildup welding coating layer) can be continuously formed under combustion synthesis in a thickness ranging from hundreds of microns (xcexcm) to several millimeters (mm). Thus, a dense buildup welding coating whose formation has been difficult with conventional technologies can be formed with reduced energy consumption within a short period of time in the present invention. That is, the present invention enables providing an apparatus for practically executing a building up coating of intermetallic compound, the intermetallic compound having high-temperature strength and corrosion resistance but necessitating difficult working, with the utilization of the high formation heat of intermetallic compound, for example, NiAl or TiAl, and providing a process for forming an intermetallic compound coating. From the material point of view, the present invention enables providing a highly functional member of intermetallic compound whose working is extremely difficult in an economic manner.
The method of the present invention is advantageous in that a coating of intermetallic compound can be formed with reduced energy consumption within a short period of time, and enables manufacturing high-performance parts of excellent high temperature heat resisting strength as final products at low cost by streamlining complex processes of the conventional working and metallurgical technologies.